ST. LOUIS — My late, great college professor, the eloquent Jack Falla, used to say, “In life, sometimes you’ve got to play hurt.”

It was more than a quaint maxim or a brush-it-off-kid bromide. It conveyed the truth that often times in life you are asked to perform at your best when you don’t feel your best. Opportunity doesn’t always align with ideal circumstances or optimum health.

Red Sox starter Clay Buchholz would be wise to heed Falla’s words. The only thing that should stop Buchholz from taking the mound on Sunday for Game 4 is the National Guard. It’s a start he needs to make with conviction, not with an eye toward a Felix Doubront bail-out after an inning or two.

The latest news from Clay Watch on Friday at Busch Stadium, site of the next three games of the World Series, was that Buchholz is still slated to take the ball in Game 4.

“He went through a throwing program today, went back to about 100 feet with some increased intensity along the way, and he’s starting Sunday,” declared Red Sox manager John Farrell.

But you get the sense that Buchholz, who missed three months with shoulder bursitis, is keeping the exit door slightly ajar, just in case.

Buchholz’s biggest problem on Sunday might not be his shoulder, but his head. He has to get it out of his mind that he can’t be effective at less than 100 percent.

He doesn’t have to be the righthanded wrecking ball he was when he jumped out to a 9-0 record with a 1.71 earned run average before the shoulder ailment sidelined him in June. If he’s 75 percent of that pitcher, he can help the Sox in this series.

Buchholz’s willingness and capability to pitch through discomfort could be the hinge on which this series turns. The Sox already pushed Buchholz back from Game 3 after he revealed that he was experiencing tightness in the rehabbed shoulder.

Jeremy Renner doppelganger Jake Peavy gets the ball on Saturday in Game 3. That sets Peavy, who is trying to exorcise past postseason demons, on course to pitch a potential Game 7.

Buchholz is one of the most earnest, polite professional ballplayers you’ll come across. He is honest almost to a fault.

That’s where he gets in trouble because he will speak openly and frankly about not feeling 100 percent.

It’s always dangerous to project what another person is feeling inside their body or what level of discomfort they’re able to tolerate, but on Wednesday, Buchholz spoke of being at “90 percent” for his start in Game 4.

“I’m going to do everything I can to . . . be well enough to go out there and give the team a chance to win,” he said. “If that’s me going out there at 90 percent, I’ll go out there at 90 percent. But if it comes down to the wire, and I’m going to run out there and not help the team win, there is no reason for me to run out there.”

Buchholz makes taking the mound at 90 percent sound like doing it with Bubonic plague.

What percentage was Curt Schilling at in the 2004 American League Championship Series or the World Series, when his ankle was stitched together like a patchwork quilt?

The precedent for playing through pain has already been set in this World Series by the Cardinals’ Carlos Beltran, who crashed into the right-field wall at Fenway Park to rob David Ortiz of a grand slam in Game 1 on Wednesday. Beltran left the game with a rib contusion and was checked out at a hospital.

One night later, with the aid of a pain-killing injection, Beltran was in the Cardinals lineup, helping them to a 4-2 win.

Beltran was asked what it would have taken for him to not play in Game 2. He said somebody would have had to kill him.

The hyperbolic and a cringe-inducing choice of words aside, it was a sincere expression of how deep Beltran’s desire was to play in the biggest games of the year.

If Buchholz doesn’t display the same determination and grit pitching through shoulder tightness, then his handle-with-care label is going to become permanent.

He already took a public relations hit with Conflicting Diagnosis Theater, a drama featuring Buchholz and the team’s dueling takes on his status with the shoulder bursitis.

Whatever percentage he was at when he returned, Buchholz was good in September. In his final four starts of the regular season, he was 3-1 with a 1.88 ERA. Opponents hit just .217 against him, and he struck out 15 batters and walked seven in a combined 24 innings.

This postseason, his numbers don’t look impressive.

In three starts, Buchholz has not pitched into the seventh inning. He has posted a 5.40 ERA and allowed 19 hits in 16⅔ innings. Opponents are batting .284 against him.

But those numbers are skewed by Game 2 of the American League Championship Series against Detroit, when he surrendered five earned runs in 5⅔ innings.

In his last outing, Game 6 of the ALCS, Buchholz pitched five shutout innings before being lifted two batters into the sixth. He was charged with a pair of runs when Franklin Morales gave up a single.

The Sox have won without Buchholz before. They went 49-33 without him when he missed three months. But it will be hard to win the World Series without him.

Sometimes you have to play hurt. Pitching in the World Series is one of those times.

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